feat: LicenseGuard 中间件 + Flutter license model/repo (21D+21E)

21D — 后端:
- Claims 新增 LicenseExpiresAt (*int64 unix 秒)，写入 JWT 避免每次查库
- middleware/license_guard.go: CalcLicensePhase / LicenseGuard / GetLicensePhase
  - grace(0-7d): 允许通行
  - readonly(7-15d): 拦截非 GET 写操作 → 403
  - locked(15d+): 全部拦截 → 403
- auth.go: issueTokens 在 JWT 中嵌入 license expires_at；Login 检查 locked 拒绝登录
- router: license/* 路由豁免 LicenseGuard（锁定时仍可激活/查状态）

21E — Flutter 前端:
- models/license.dart: 新 LicenseInfo，含 phase/maxDevices，去掉旧 activatedAt
- core/device/device_id.dart: 持久化 UUID-v4 作为设备 ID（SharedPreferences）
- repositories/license_repository.dart: getInfo/activate/deactivate，激活时携带设备信息
- providers/license_provider.dart: 改接 LicenseRepository
- settings_screen.dart: activatedAt 改为显示 maxDevices

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/middleware/auth.go

@@ -10,16 +10,18 @@ import (
 )
 
 type Claims struct {
-	UserID uint64 `json:"user_id"`
-	ShopID uint64 `json:"shop_id"`
-	Role   string `json:"role"`
+	UserID            uint64 `json:"user_id"`
+	ShopID            uint64 `json:"shop_id"`
+	Role              string `json:"role"`
+	LicenseExpiresAt  *int64 `json:"lic_exp,omitempty"` // unix seconds; nil = perpetual
 	jwt.RegisteredClaims
 }
 
 const (
-	CtxUserID = "user_id"
-	CtxShopID = "shop_id"
-	CtxRole   = "role"
+	CtxUserID           = "user_id"
+	CtxShopID           = "shop_id"
+	CtxRole             = "role"
+	CtxLicenseExpiresAt = "lic_exp"
 )
 
 func JWT() gin.HandlerFunc {
@@ -43,6 +45,7 @@ func JWT() gin.HandlerFunc {
 		c.Set(CtxUserID, claims.UserID)
 		c.Set(CtxShopID, claims.ShopID)
 		c.Set(CtxRole, claims.Role)
+		c.Set(CtxLicenseExpiresAt, claims.LicenseExpiresAt)
 		c.Next()
 	}
 }

backend/internal/middleware/license_guard.go

@@ -0,0 +1,77 @@
+package middleware
+
+import (
+	"net/http"
+	"time"
+
+	"github.com/gin-gonic/gin"
+)
+
+const (
+	PhaseNormal   = "normal"
+	PhaseGrace    = "grace"    // expired 0–7 days: writable, show banner
+	PhaseReadOnly = "readonly" // expired 7–15 days: read-only
+	PhaseLocked   = "locked"   // expired 15+ days: no login
+)
+
+var (
+	graceWindow    = 7 * 24 * time.Hour
+	readOnlyWindow = 15 * 24 * time.Hour
+)
+
+// CalcLicensePhase computes the degradation phase based on expires_at.
+// nil expiresAt = perpetual license = normal.
+func CalcLicensePhase(expiresAt *time.Time) string {
+	if expiresAt == nil {
+		return PhaseNormal
+	}
+	elapsed := time.Since(*expiresAt)
+	if elapsed <= 0 {
+		return PhaseNormal
+	}
+	if elapsed <= graceWindow {
+		return PhaseGrace
+	}
+	if elapsed <= readOnlyWindow {
+		return PhaseReadOnly
+	}
+	return PhaseLocked
+}
+
+// GetLicensePhase returns the current phase for the authenticated request.
+func GetLicensePhase(c *gin.Context) string {
+	v, _ := c.Get(CtxLicenseExpiresAt)
+	ptr, _ := v.(*int64)
+	if ptr == nil {
+		return PhaseNormal
+	}
+	t := time.Unix(*ptr, 0)
+	return CalcLicensePhase(&t)
+}
+
+// LicenseGuard blocks write operations when the shop's license is expired (readonly/locked).
+// License routes (/license/*) must be mounted outside this middleware so users can
+// view status and activate a new key even when locked.
+func LicenseGuard() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		phase := GetLicensePhase(c)
+		switch phase {
+		case PhaseLocked:
+			c.AbortWithStatusJSON(http.StatusForbidden, gin.H{
+				"error": "授权已锁定，请续费或激活新授权码",
+				"phase": PhaseLocked,
+			})
+		case PhaseReadOnly:
+			if c.Request.Method != http.MethodGet {
+				c.AbortWithStatusJSON(http.StatusForbidden, gin.H{
+					"error": "授权已过期，当前为只读模式",
+					"phase": PhaseReadOnly,
+				})
+				return
+			}
+			c.Next()
+		default:
+			c.Next()
+		}
+	}
+}